This invention relates generally to electromagnetic borehole telemetry, and more particularly to efficient and controllable coupling of energy from a downhole energy source to an earth-drillstring system.
In electromagnetic borehole telemetry, it is known to couple electromagnetic energy to the earth-drillstring system by means such as toroidal coupling transformers, ferrite rod antennae, impedance matching switching amplifiers, and other similar devices, in order to provide an optimum matching of the energy source impedance to the earth-drillstring load impedance. The significant technical problem in electromagnetic transmission through and/or along the earth-drillstring system is the high attenuation of signal due to the generally high conductivity of the earth portion of the transmission path. Several approaches have included provision of repeater amplification means along the transmission path, to offset the severe attenuation problem.
Systems proposed for such intended usages include those disclosed in U.S. Pat. Nos. 2,354,887; 2,389,241; 2,411,696; 2,492,794; 3,079,549; 3,115,774; 3,048,561; 3,793,632; 3,967,201; 4,057,781; 4,181,014; 4,348,672; and 4,691,203.
The large number of patents and extensive published literature on the subject attest to the large amount of work done in this field and the difficulties of achieving the desired results. Despite the extensive work shown by the prior art, there has been very little commercial success obtained. One significant explanation is that too much was expected from each chosen approach, and that when the limitations of the physical problem prevented full realization of the goals, the effort was dropped in favor of other approaches. Also, it is believed that the complexities of certain approaches, when reduced to practice, resulted in poor equipment reliability in downhole drilling environments, excessively high initial equipment cost, and excessively high operation costs.